The present invention relates to a method of making large-scale or large-surface area precision structures in or on flat glass and also to an apparatus for performing that process.
Flat glass provided with large-scale precision structures is required for precision applications, especially in the optical glass field. This type of glass includes, for example display panels of new generation flat display screen devices (Plasma Display Panels (PDP), Plasma Addressed Liquid Crystal (PALC)). Micro-channel structures for control of individual lines or columns, which extend over the entire active display screen width or height and in which a plasma is ignited by electric discharge, are provided in this flat display screen glass. The boundary of each individual channel on both sides of it is provided by a rectangular crosspiece whose width is as small as possible (i.e.&lt;100 .mu.m). In order to obtain a sufficient discharge volume, the height of the crosspiece is substantially larger than its width. The spacing of the crosspieces should be as small as possible. Currently typical values of between 360 .mu.m and 640 .mu.m are achieved in small scale production. The height of the crosspieces amounts to from about 150 .mu.m to 250 .mu.m at a width of from 50 .mu.m to 100 .mu.m. Two electrodes for plasma ignition extend through each individual channel bounded by the crosspieces in plasma addressed liquid crystal (PALC), while one electrode for plasma ignition extends through each individual channel bounded by the crosspieces in plasma display panels (PDP).
During the structuring of this flat display screen glass, which for example is a 25"-PALC screen of a size of 360 mm.times.650 mm, the exact lateral dimensioning, relative positioning and reproducibility of the channel and thus the stability of the forming tool are crucial because of the later positioning of the electrodes. With a method based on hot shaping by means of a conventional Chromium-Nickel-Steel tool, the thermal expansion coefficient amounts to about 12.times.10.sup.-6 /K. For example, for a tool length of about 360 mm, as required for a 25"-PALC display screen, this always causes a length change of about 4 .mu.m per K temperature fluctuation. Considering that the required positioning accuracy of the electrodes in the micro-channels is in the range of .+-.10 .mu.m, a temperature fluctuation of .+-.2.5 K can cause considerable problems.
The permissible temperature fluctuations are considerably reduced in the larger display screens, for example 42" display screens.
The problems are similar with other applications of flat glass with large-scale precision structures.
It is known to use hot shaping methods with suitably structured forming tools for making structures in flat glass.
Existing specifications limit however the possible applications of conventional hot shaping methods, such as rolling or pressing, for making of large-scale precision structures.
Conventional hot shaping methods have the following disadvantages:
When a contact between the glass and a press or roll tool acting as the forming tool occurs only for a short time, i.e. prior to solidification the work tool is removed from the glass, because of flow of the glass structure, a strong rounding occurs after this contact. PA1 In a long-duration contact which is used in a cold-pressing method, intolerable stresses arise because of strong temperature differences and different thermal expansions of the tool and glass. PA1 Since the forming tool is heated completely in a conventional hot shaping, in order to achieve a sufficient surface temperature on the contact surface for the glass, high non-reproducible temperatures occur in the required precision range of .+-.2 K ( with typical work tool steels and glass surface areas required by the specifications), which lead to intolerable deformation of the work tool. PA1 A higher tool wear, which requires a replacement of the forming tool, occurs during the making of structured glass with reduced structure radii. PA1 a) supplying a paste-like material forming the structures to a structuring surface of a forming tool; PA1 b) applying the paste-like material by a single pressing of the work tool to the flat glass and hardening the applied paste-like material; and PA1 c) melting the material applied on the flat glass by a local exterior heating of the structuring surface of the forming tool until a surface depth predetermined by the height of the structures has reached the required process temperature. PA1 a device for supplying a paste-like material forming the structures to a structuring surface of a forming tool, PA1 means for applying the paste-like material by a single pressing of the forming tool to the flat glass and means for hardening of the applied paste-like material; and PA1 means for melting the material to be applied to the flat glass by local heating of the structuring surface of the forming tool with the applied paste at the required process temperature by a suitable exterior heat source.
It is more difficult to prevent adherence of the tool to the glass in both methods with increasing tool temperature.
An additional essential requirement of the method of making these glasses is the maintenance of a stable production process, in which the local distribution and form of the structures are kept extremely constant. Additional limitations of the conventional hot shaping are as follows:
On account of these limitations and disadvantages in the current methods of making of structures by hot shaping methods these methods have not been used up to now for making large-scale precision structures. Instead of that structures of this type are provided currently according to the state of the art on flat glass be means of screen printing methods, in which the boundaries forming the structures, the crosspieces in the above-described flat display screens, are pressed in the flat glass layer-wise by means of a glass solder. The shaping occurs by means of suitable masks applied to the glass solder, which have openings for the structures to be formed, the crosspieces for example. The masks must thus be applied for each pressed layer. About 10 to 15 layers are necessary for the above-described crosspieces in the flat display screen glass, which are about 200 .mu.m high and on average about 70 .mu.m wide, based on the about 30 .mu.m thick electrodes applied in 2 to 3 layers between the crosspieces.
These screen printing methods require a considerable process engineering expense, because hardening must occur after the application of the mask on the glass solder and the filling of the mask openings. The solder must also be adjusted or selected in regard to its physical and chemical properties to fit the glass base substrate (e.g. thermal expansion coefficient) and the respective application (e.g. resistance to plasma ignition) for contacting the glass solder on a suitable glass substrate. A certain processing time of typically from 5 to 10 minutes at 450.degree. C. is required for hardening this type of glass solder according to the application, which results in a comparatively high processing time with 15 layers, so that the known screen printing process is limited to prototype production and is uneconomical for later series production.